The present invention relates to the field of steering mechanisms, and in particular to a power-assisted steering system having a gear mechanism with a gear and a mating gear.
Power-assisted steering is regularly used in motor vehicles to assist with manual steering movements that a driver performs on the steering wheel. Power-assisted steering systems include hydraulics to minimize the steering forces and reduce steering, especially at low speeds or when the vehicle is stationary. Power-assisted steering improves driver comfort, especially in parking and maneuvering, and in city traffic. In addition, steering systems are being developed that are electrical rather than hydraulic systems.
FIG. 1 is a pictorial illustration of a power-assisted steering system 100 with electromechanical steering assistance. FIG. 1 depicts a steering wheel 102 connected via a steering column 104 to a gear mechanism that has a worm gear 106 and a worm 108. In addition, a worm-drive electric motor 110 is coupled to the gear mechanism. This gear mechanism is connected via a drag link 112 to a steering rack 114 of the motor vehicle. A tie rod 116 is coaxial with the steering rack 114.
Gear mechanisms generally transmit a rotary movement of one shaft to another, which frequently occurs with conversion of a torque. Through teeth meshing with one another, positive connection between the shafts is provided and gear mechanisms ensure compulsory, non-slip transmission of the rotary movement, or torque.
Gears with involute tooth design are in almost exclusive use in mechanical engineering. In an involute tooth design, the effective profiles of the tooth faces (i.e., the tooth face profiles that come into contact with one another when the teeth mesh and through which force is transmitted) are involutes of a circle. That is, they describe a curve obtained by constructing a tangent at points of a circle and deducting on the tangents the length of the arc from the point of contact of the tangent with the circle up to a certain fixed point of the circle. In the case of externally toothed gears, the effective profiles of an involute tooth design are convex.
Gears with involute tooth design can be made in a relatively simple and precise manner. An advantage of this tooth geometry is that various tooth shapes and axis spacings can be made with the same tool by shifting the profile. In operation, gears with involute tooth design are distinguished by the fact that the direction and the magnitude of the tooth normal force is constant during the engagement of the teeth, resulting in uniform loading of the entire mechanism, in particular of the bearings of the mechanism.
Gear mechanisms have a variety of uses. They are used both in precision technology and in vehicle construction, for example in steering assistance systems.
FIG. 2 depicts a worm drive 200 with a spur-gear-shaped worm gear 202 and a worm 204 engaging with the worm gear 202, each with involute tooth design. In operation (i.e., when the teeth of the worm 204 and the worm gear 202 mesh with one another) the teeth of the worm gear 202 make contact with the teeth of the worm 204 at a point 206. This contact leads to a high loading of the teeth at this point 206, which, depending on the material pairing, can lead to severe wear and, in the extreme case, overloading of the teeth. The load-bearing capacity of gears with a point contact is thus limited.
FIG. 3 depicts a worm gear mechanism 300 having a worm gear 302 and a worm 304 that engages the worm gear 302. In contrast to the worm gear mechanism depicted in FIG. 2, the worm gear 302 is globoidal rather than cylindrical in shape. As a result, the worm gear 302 contact with the worm 304 is along a line 306 over the width of the teeth, so that the load transmitted by the teeth is distributed over a larger area. This reduces the loading per unit area of the individual teeth, so that the load-bearing capacity of the gears is increased. As a consequence, both wear and the danger of overloading of the teeth are decreased.
Machining is required to make a globoidal worm gear, since globoidal worm gears have undercut regions. However, machining increases the cost of manufacturing in comparison to other techniques for making gears, such as for example injection molding. In addition, the assembly of worm gear mechanisms with globoidal worm gears is more expensive because the worm gear can only be mounted in the radial direction, and not in the axial direction. Radial insertion of the worm gear requires more space than axial insertion, and may damage the worm gear if the worm gear is not moved toward the worm in the correct angular position. This is particularly true when the worm gear is made of a material with lower strength than the material of the worm. In addition, the worm and the worm gear must be positioned precisely relative to each other so that the teeth mesh properly. Another drawback is that if the angle between the axes of the worm and the worm gear is not equal to 90°, the worm gear must be made less globoidal. As a result, the linear contact area 306 becomes smaller, which in turn has a negative effect on the load-bearing capacity.
German Patent DE4107659A1 discloses a worm gear mechanism in which the worm and the worm gear have their force-transmitting contact surfaces shaped to provide low-noise operation. The tooth bases are each concave in shape and the tooth tips are convex in shape. An involute middle tooth part is provided in each case between these concave and convex portions. However, the use of the involute middle tooth part leads to the situation that only a point contact is provided between the meshing teeth in the involute region. As a result, this known worm gear mechanism does not have adequate bearing capacity for high loads throughout their meshing region.
Therefore, there is a need for a power-assisted steering system that includes gears with improved coupling.